1. Field of the Invention
The present invention relates to shape memory polymers and, more particularly, to shape memory polymers featuring reversible actuation capability under ambient stimulus for integration with apparel.
2. Description of the Related Art
Shape memory materials are those materials that have the ability to “memorize” a macroscopic (permanent) shape, be manipulated and “fixed” to a temporary and dormant shape under specific conditions of temperature and stress, and then later relax to the original, stress-free, condition under thermal, electrical, or environmental command. This relaxation is associated with elastic deformation stored during prior manipulation. Shape memory materials have aroused great attention by scientists and engineers due to their capacity to remember two shapes at different conditions.
The most prominent and widely used shape memory materials currently are shape memory alloys (SMAs). Their shape memory effect comes from the existence of two stable crystal structures: the high temperature-favored austenitic phase and low temperature-favored (and “yield-able”) martensitic phase. Downsides that limit their application include limited recoverable strains less than 8%, inherently high stiffness, high cost, comparatively inflexible transition temperature, and demanding processing and training conditions. Such limitations have provided motivation for the development of alternative materials, especially polymeric shape memory materials. Polymeric materials are intrinsically capable of a shape memory effect, although the mechanisms responsible differ dramatically from those of metal alloys. In SMAs, pseudoplastic fixing is possible through the martensitic de-twinning mechanism, while recovery is triggered by the martensite-austenite phase transition. In contrast, shape memory polymers achieve temporary strain fixing and recovery through a variety of physical means, while the underlying extensibility is derived from the intrinsic elasticity of polymeric networks.
The shape memory effect in polymers can take two quite distinct forms: one-way or two-way shape memory. In the one-way shape memory case, the cycle is started at low stress and high temperature, at which point the stress ramped to deform the sample, and following which steps of cooling under load and then unloading at low temperature reveal the quality of shape “fixing”. Finally, reheating the sample to the original temperature leads to strain recovery. In contrast, two-way shape memory features reversible actuation at a single applied stress, as shown in FIG. 1b. The two-way shape memory case is the one most easily exploited for actuation purposes of interest for the present invention, while one-way shape memory is required for controlled, one-time deployment events, such as expansion of slender medical device or unfolding of a complex structure. In the present application, one-way and two-way shape memory are referred to as 1 W-SM and 2 W-SM, respectively. However, no 2 W-SM polymer has been reported to date that would allow ambient (or environmental) stimulus at temperatures in the 20-40° C. range and with amenability for integration with fabric.